Method and apparatus for analyzing seismographic records



March 8, 1949. J. EwHAWKlNS 2,463,534

' METHOD AND APPARATUS FOR ANALYZING S EISMOG'RAPHIC RECORDS Filed Jan.11, 1947 V 3 Sheets-Sheet 1.

"GE R uscnmism AMPLIFIE AMPLIFIEI: j I

:zmmne 3' 3 V 4a f" INVEN TOR. JAMES E. HAWKINS ATTORNEY March 8, 1949.v Y J. E. HAWKINS 2,463,534

METHOD AND APPARATUS FOR ANALYZING v SEISMOGRAPHI-C RECORDS Filed Jan.11, 1947 3 Sheets-Sheet 2 J. E. HAWKINS METHOD AND APPARATUS FORANALYZING March 8, 1949.

:5 Sheets-Sheet 5 Filed Jail. 11, 1947 SEISMOGRAPHIQ RECORDS FIG. 3

FREQUENCY FIG.'6

FIG. 5

i I wan-:9 our TIME-4 "STEP our TIME FIG. 7*

FIGS,

l h-STEP OUT TIME F-STEP OUT TIME- 1 5 &W K T MW A VH m5 8 E M A J m MnRN CAA m 6 R M m 8 8 il 2 I--- 7 .2 an M mm IAEEH w m 9 M G n: a l 2ATTORNEY Patented Mar. 8, 1949 METHOD AND APPARATUS FOR ANALYZINGSEISMOGRAPHIC RECORDS James E. Hawkins, Tulsa, Okla., assignor toSeismograph Service Corporation, Tulsa, Okla., a

corporation of Delaware Application January 11, 1947, Serial No. 721,559

11 Claims.

This invention relates to a method and apparatus for the analysis ofseismographic records of the type produced in practicing seismic methodsof geophysical prospecting and is a division of an application forLetters Patent of the United States, Serial No. 663,205, filed April 18,1946.

When seismic methods are employed in geophysical exploration, motions ofthe earth are initiated by suitable means such, for example, asexplosions produced beneath the earths crust to create elastic waveswhich are transmitted through the earth. The initial disturbance createdby the explosion is not a wave in the ordinary periodic sense of wavephenomena. However, in traveling through the earth, the disturbanceacquires a periodic character in that it becomes a short pulse or dampedwave train. Any

discontinuities or variations of structure within M the earth willreflect and refract this damped wave train or a portion thereof so thata record made at the receiving point will comprise a number of arrivingwaves each derived from the original pulse, and each differing from theothers in time of arrival, magnitude or both. The desired information isderived from the record by determining the instant of each successivearrival.' It is apparent that for certain geological structures a verycomplex set of waves are received and recorded at the recorder stationso that the resultant record of the visual inspection type is extremelycomplex in character. The determination of the instant of arrival of thevarious waves is arrived at by record interpretation and the informationto be gained from the record is sometimes so unreliable andcontradictory that the record is of little value. Such records,particularly where made in regions that are not exceptionally favorable,are very diflicult of solution by visual inspection.

It has been suggested that instead of recording the receipt of thevarious waves on a record for visual inspection, that a record which isphonographically reproducible in a manner described in U. S. LettersPatent No. 2,051,153-Rieber, be employed. Records of this type may be ofthe variable density or variable area type so as to lend themselves verywell to scanning in the same general fashion as is employed in thescanning of talking pictures or they may be magnetically recorded asWell. It has been the practice in seismic explorations wherephonographically reproducible records are employed, to record all thevibrations received at the geophones and to dispense with the employmentof selective filters as is the case when the visual inspection type ofrecord is initially produced. The phonographically reproducible recordssuch as variable area or variable density records of the seismic eventscan then be leisurely analyzed by repetitive reproduction thereof. It iscommon practice in analyzing such phonographically reproducible seismicrecords to finally produce therefrom a visual inspection type of recordwhich has the undesirable wave components eliminated therefrom andessentially contains only the desirable seismic information.

One of the problems involved in the production of an acceptable visualinspection or oscillographic trace type of seismic record from aphonographically reproducible record such as variable area or variabledensity record which contains all of the waves received by the geophonesis that of filtering from the initially recorded energy those componentswhich may be attributed to noise or other sources of wave energy notpertinent to the ascertainment of the geological structure. The usualpractice in eliminating some of the undesirable components has been thatof employing relatively complicated band pass filters having adjustableupper and lower limits or combinations of filters, one or more of whichmay be adjustable for the purpose of suppressing the noise energy duringthe translation step from the initially recorded signal to a visualinspection type of trace. In general, this prior practice has severalobjectionable features among which were the complexity of the filteringequipment, the number of time-consuming adjustments which are requiredto obtain the desired pass characteristics through the filter networksand the lack of accuracy in controlling the limits of the passcharacteristic through the filter networks. It is desirable, therefore,to provide a simple arrangement for selective filtering by which theundesirable energy initially recorded in connection with geophysicalexploration by seismic methods may be eliminated and a final recordproduced containing substantially only desired information.

Accordingly it is an object of the present invention to provide a newand improved method and apparatus for analysis of seismographic records.

It is another object of the present invention to provide a new andimproved method and apparatus for analyzing seismographic records inwhich selective filtering of information contained on the initial recordis accomplished by varying the motion of the initial record while thefinal record therefrom is being produced.

Still another object of the present invention is to provide a new andimproved method and apparatus for converting seismograms of thephonographically reproducible type such as of the variable area orvariable density type to visual inspection type traces in which thespeed of movement of the seismogram during the reproduction operation isvaried in accordance with a predetermined time function.

It is a further object of the present invention to provide apparatus foranalyzing seismograms by converting variable area or variable densitytracks to visual inspecion type traces which apparatus will accommodateseismograms of varied lengths.

Further objects and advantages of the present invention will becomeapparent as the following description proceeds and the features ofnovelty which characterize the invention will be pointed out withparticularity in the claims annexed to and forming a part of thisspecification.

For a better understanding of the present invention reference may be hadto the accompanying drawings in which:

Fig. 1 is a schematic view illustrating apparatus embodying the presentinvention;

Fig. 2 is a sectional view of one of the components of the apparatus ofFig. 1 taken on line 22 of Fig. 1;

Fig. 3 is a curve diagram to aid in understanding the operation of thepresent invention;

Fig. 4 is an elevational view illustrating a modification of a portionof the apparatus shown in Fig. 1;

Figs. 5, 6, 7 and 8 represent portions of final records of the visualinspection or curve type obtained by analysis in accordance with thepresent invention; and

Figs. 9 and 10 are schematic views illustrating still othermodifications of the present invention.

The present invention is concerned with the analysis of a seismographicrecord of the phonographically reproducible type. It will be apparent asthe following description proceeds that the invention is equallyapplicable to analyzing any type of phonographically reproduciblerecords. In order to specifically disclose the invention, the variablearea type of record has been illustrated in Fig. 1 Where there isdisclosed a strip of film or record I having edge perforations I I.Recorded on film II] is the seismic information obtained from aplurality of geophones during a seismic prospecting operation whichinformation appears on film III as variable area tracks or traces I2, I3and I4, respectively. A sinusoidal timing wave also of the variable areatype is indicated at I5. The record or film I0 is of the usual type inwhich the area thereof not covered by the variable area records I2, I3,I I and I5 is transparent.

In order to analyze the information contained on the initial record I0,which incidentally contains all of the Wave information received by thegeophones, the film or record Ill is inserted in the reproductionapparatus or mechanism of the present invention schematically shown inFig. 1 of the drawings which comprises a rotatable drum I6 having at oneend thereof a suitable sprocket arrangement including the teeth I! forengagement with the edge perforations II of the film or record Ill and aroller I8 spaced from the drum IE but having its axis parallel with theaxis of the drum IS. The film or record In, with its ends splicedtogether as indicated at I9 in Fig. 1 of the drawings to form an endlessbelt or loop,

encircles the drum I6 and the roller I8 with the edge perforations IIengaging the teeth I1 so as to be driven as a belt upon rotation of drumI6. In order that the reproduction apparatus comprising the drum I6 andthe roller I8 can accommodate various film lengths, the roller I8 isrotatably supported by a member 20 adjustably mounted for movementtoward and away from the drum It by means of a manually rotatable screw2| threadedly engaging with a stationary support 22. With thisarrangement the reproduction mechanism of the present invention isadapted for reproducing records of difierent lengths, it being merelynecessary to adjust the screw 2| when it is desired to accommodate arecord such as the record II] in the form of an endless belt whoselength is greater or less than that of the specific record or film II].

To permit reproduction of the initially recorded variable area seismictraces I2, I3 and I4 in the form of curves which can be read andanalyzed by visual inspection, the rotatable drum I6 is preferablyconstructed as a hollow, transparent drum having an open end into whichis inserted a statoinary detachable photo-electric cell assemblygenerally designated by the reference numeral 23 and described ingreater detail hereinafter. The drum It may be formed from anytransparent material such as glass, methyl methacrylate or the like andhas its closed end fastened to a rotatable shaft 24 as indicated at 25.This shaft is drivingly connected to a synchronous driving motor 26through a change-speed gear mechanism schematically indicated at 21. Itwill be apparent that with this arrangement the transparent drum I6 maybe rotated around the photo-electric cell assembly 23 at any desiredsynchronous speed merely by pushing or pulling on a speed control knob28 associated with the change-speed gear mechanism 21.

It will also be understood that the reason for employing the transparentdrum I6 is to enable light rays from a suitable source to be fed throughthe film record I0 and the drum I 6 to the photo-electric cell assembly23. Accordingly there is provided a source of light 29 spaced a shortdistance away from the drum I6 on a line perpendicular with the axis ofthe drum and intersecting a point substantially midway between the endsthereof. Interposed between the drum I6 and the light source 29 is amask 30 having a longitudinally extending slit 3| and a long cylindricallens 32. Light from the source 29 in passing through the slit 3| in themask 30 and the lens 32 whose axis is at right angles to the directionof motion of the film or record It as indicated by the arrow in Fig. 1of the drawings is directed as a narrow horizontal beam through the filmIll and the transparent drum I6 to the photoelectric cell assembly 23 inthe manner clearly shown in Fig. 2 of the drawings. It will be apparentthat part of this beam of light will be intercepted by the variable areatraces I2, I3, I4 and I5 on the film or record I 0.

In order that the variations in area of the traces I2, I3, I4 and I5which intercept the light beam from source 29 may be converted toelectrical energy the photo-electric cell assembly 23 is provided.Briefly, this assembly comprises a cylindrical container 33 supported bya member 34 from a fixed support 35 and a plurality of photo-electriccells 36, 31, 38 and 39. The photoelectric cells 36, 37, 38 and 39 aresupported Within the container 33 in spaced relationship so as to beassociated with the variable area traces I2, I 3, I4 and I5,respectively, on the film or record III. A plurality of lenses 40, 4|,42 and 43 are disposed between the associated photo-electric cells 36,31, 38 and 39, respectively, and the light source 29 so as to focus thebeams of light, part of which are intercepted by the variable areatraces I2, I3, I4 and I5 on the associated photo-electric cells. Thecylindrical container 33 is also provided with a longitudinallyextending opening 44 through which the light from source 29 may pass onits way to the photo-electric cells 36, 31, 38 and 39. As thus arranged,the amount of light falling on a particular one of the photo-electriccells 36 to 39 inclusive depends upon the amount of light passed throughthe clear portion of the film upon which the narrow horizontal lightbeam from the source 29 is focused. Consequently the light falling onthe photo-electric cells depends upon the instantaneous amplitudes ofthe variable area traces on the record I 0, so that the electricaloutput of a particular photo-electric cell is proportional to the lightfalling upon it, or in other words, is proportional to the instantaneousamplitude of the associated variable area trace.

It will be understood that the changes in light received by thephoto-electric cells 36, 31, 38 and 39 by virtue of the variable areatraces I2, I3, I4 and I5, respectively, are converted to electricalchanges in the output circuits of the respective photo-electric cells.As illustrated in Fig. 2 of the drawings the photo-electric cell 36 hasconnected thereto a pair of conductors 45, which conductors lead to theinput circuit of an amplifier 46. Similarly the photo-electric cell 31has connected thereto a pair of electrical conductors 41 which extend tothe input circuit of an amplifier 48. Also the photo-electric cell 38has connected thereto a pair of conductors 49 which are connected to theinput circuit of an amplifier '50. The photo-electric cell 39, on theother hand, which is adapted to receive light intercepted by the timingtrace I 5 of the variable area type, has

connected thereto a pair of electrical conductors 5| which are connectedto the input circuit of an amplifier 52.

In order that the amplified electrical signals which appear at theoutput sides of amplifiers 46, 48 and 5|) may be converted to curves ona record which may be visually inspected, there is provided a recordingcamera generally indicated at 53. Briefly, this camera comprises amovable strip of sensitized paper generally indicated at 54, which ismovable from a suitable supply spool not shown to a take-up spool 55 bymeans of a roller 56 in the form of 'a sprocket having teeth forengaging edge perforations 51 in the sensitized strip 54. A suitabletape or strip driving motor 58 connected to the sprocket or roller 56causes the paper to move in the direction of the arrow and to betransferred to the take-up spool or roller 55. Preferably the sensitizedpaper or strip 54 is of the type which has a record produced thereonwhen subjected to a beam of light and subsequently developed in the samemanner as a camera film. For the purpose of illustratingthe presentinvention, a plurality of curves or traces of the visual inspection type59, 60 and 6| are illustrated as being produced on the sensitized strip54. In practice, however, these curves or traces 59, 60 and 6| would notbe visible in the manner indicated in Fig. 1 of the drawing until afterthe film has been developed.

In producing seismograms it is customary to include a time recordthereon in the form of lines which extend across the face of theseismogram and which are spaced from each other by intervals of timesuch as one-tenth or one-hundredth of a second. As mentioned above, atime record appears on the original record or film III in the form of avariable area trace I5. In order that the time information contained inthe trace I5 may be reproduced on the sensitive strip 54 in the form ofuniformly spaced timing lines generally indicated at 62 simultaneouslywith the production of the traces 59, 68 and BI, the output of theamplifier 52, which is connected to the timing photo-electric cell 39,is connected to supply a synchronous motor 64 With electrical energy. Itwill be understood that since the area of the timing trace I5 variesperiodically and as illustrated sinusoidally, the outputof amplifier 52will be an alternating current or potential having a frequency dependentupon the speed of rotation of the drum I6. The synchronous timing motor64 is connected to drive suitable means for applying timing lines to thesensitized paper or strip 54. In the illustrated arrangement, the timingmotor 64 is connected to drive a drum 65 which is provided with aplurality of uniformly spaced slits 66 extending longitudinally of thedrum through which light from a source 61 within the drum 65 may betransmitted. This light is adapted to pass through a slit 68 in a maskor screening plate 69 and then through a lens III to produce a recordline such as 62 on the sensitized strip 54. The plate or mask 69 insuresthat the light from only the particular slit 66 in the drum 65 which'isin registry therewith can reach the sensitized paper 54 through the lens10.

In order to record the visual inspection type traces 59, 68 and 6| onthe sensitized stri 54 of the recording camera 53, this camera isfurther equipped with a plurality of oscillographic elements generallyindicated at II, 12 and I3. Each of the oscillographic elements II, I2and I3 may be of any general well-known type such, for example, as acoil rotating in a magnetic field and bearing a mirror I4 whereby lightfrom a plurality of light sources I5 passing through appropriate lensesI6 impinges against the mirrors I3 for reflection through the lens IIIto the sensitized strip 54. The coil of oscillographic element 'II isconnected to the output side of the amplifier 36 through a suitablefilter 88. Similarly the coil of the oscillographic element I2 isconnected to the output side of the amplifier 48 through a suitablefilter 8|. Also the coil of the oscillographic element "is connected tothe output side of the amplifier 50 through a suitable filter 82. Thefilters 80, 8| and 82, as will become apparent as the followingdescription proceeds, act to prevent some undesired signal energycontained in the variable area traces I2, I3 and I4 recorded on film orrecord I6 from appearing in the corresponding visual inspection traces59, 66 and 6|, respectively, recorded on the sensitized strip 54.

With the arrangement described thus far, it will be apparent that thevariable area traces I2, I3 and I4 appearing on the record ID areconverted by photo-electric cells 36, 31 and 38, respectively, intoelectrical signals which are amplified and filtered and then supplied tothe oscillographic elements II, I2 and I3 to produce correspondingvisual inspection traces 59, 60 and 6| on sensitized strip 54.Simultaneously with the recording of visual inspection traces 59, '60and 6|, timing lines 62 are formed on the sensitized strip 54 incorrespondence with the timing track I5 on the original record or filmI0. since the timing track 15 and the seismic wave tracks l2, l3 and 14are initially recorded on the same film or record in, the proper phaserelationship between the timing line 62 on the sensitized strip 54 andthe visual inspection traces 59, B and BI will be maintained regardlessof the speed at which the synchronous motor 26 drives the drum IEthrough the change-speed gear mechanism 21.

In order to prevent the undesired energy components recorded by thevariable area tracks l2, l3 and M (which are due to noise and otherundesirable sources of wave energy received by the seismic detectorswhen the initial recording on the film ID was made) from appearing inthe traces 59, 60 and BI on sensitized strip 54, the filters 8!], 8i and82 preferably have a fixed frequency passband characteristi or a passband characteristic which is symmetrically adjustable about a fixedcenter frequency. For example, these filters may have the fixedcharacteristic shown by the curve A in Fig. 3 of the drawings whereenergy having a frequency of between 30 and 60 cycles only istransmitted by the filters. Alternatively, they may have acharacteristic ad- J'ustable between the limits illustrated by theresponse curves A and B in Fig. 3 of the drawings. In the latter case,the filters should have a fixed center frequency with upper and lowerfrequency limits symmetrically adjustable relative to the fixed centerfrequency.

In analyzing the seismographic records in accordance with the presentinvention, the variable area tracks appearing on the film or record IDare converted to visual inspection type traces on the sensitized strip54 in a manner which will be fully apparent from the above detaileddescription. The analyzer, by visual inspection of the traces appearingon sensitized strip 54, will be able to tell that there is undesirablesignal energy of certain frequencies contained in the traces. Suppose,for example, that an examination of the traces 59, 60 and 6| made byrotating the drum I 6 at the normal speed indicated that undesirablesignal energy having a frequency of about 40 cycles is present and thatthe desired energy is of higher frequency. If the filters 80, 8| and 82have a fixed pass band characteristic such as represented by the curve Ain Fig. 3 of the drawings, this undesirable signal energy having afrequency of about 40 cycles contained in the traces recorded on thesensitized strip 54 can be eliminated from a new recording by adlustinthe change-speed gear mechanism 21 to rotate the drum IE3 at a subnormalspeed. This has the effect of shifting the entire frequency spectrum ofthe signals picked up by the photo electric cells 36, 31 and 38downwardly. Consequently by lowering the speed of rotation of the filmor record ID, the undesired 40 cycle energy can be changed to 25 cycleenergy, for example. The filters 80, BI and 82 which have the pass bandcharacteristic of the curve A of Fig. 3 of the drawings will not passthe 25 cycle energy and hence the undesired 40 cycle signal energy isexcluded from the oscillographic traces 59, B0 or Bl which are producedon the sensitized strip 54. It will be apparent that if the undesirablesignal energy has a frequency higher than that of the desired signalenergy it may be eliminated by adjusting the change-speed gear mechanism2'! to drive the drum IE at a speed above normal, thereby converting,for example, the 40 cycle signal energy to '75 cycle energy which isexcluded by the filters 85, BI and 82.

If in a given case, the desired signal energy is in a frequency banddisposed between unclesired signal energies having frequencies bothabove and below the frequency of the desired energy, the frequency passband characteristics of the filters 80, BI and 82 may be expanded andcontracted about a fixed center frequency fc to exclude the undesiredenergy. By fc is meant the mean frequency which is equal to vjifz Wheref1 and f2 are the lower and upper cut off frequencies respectively of aband pass filter (see T. E. Shea, Transmission Networks and WaveFilters); In this case the change-speed gear mechanism 21 is employed inorder to produce correspondence between the frequency of the desiredsignal energy and the fixed center frequency of the filters 80, 8! and82.

With the described arrangement, a selective filtering of the informationrecorded during a seismic surveying operation is normally obtainedmerely by using filters having a fixed frequency pass bandcharacteristic and varying the speed of rotation of the drum I 6 drivenby the synchronous motor 26 through a change-speed mechanism 21. It isapparent that this method is superior to the method of varying theconstants of the filter since by varying the speed of rotation of thedrum l6 greater flexibility and simplification of operation is obtained.From the foregoing explanation it will be understood that in practicingthe present improved method, the desired information from a seismicsurvey is obtained by using only one explosive charge and recording allthe vibrations picked up by the geophones which would be of any possiblevalue in the interpretation of the record obtained in any eographicarea. Thereafter, reproductions of this record are made by means of theapparatus described above by selectively varying the speed of rotationof the drum 16 through manipulation of the knob 28 of the change-speedgear mechanism 21. The successive records obtained on the sensitizedstrip 54 by performing the selective filtering operation described aboveenables the analyzing group to obtain the desired seismic information,which in turn permits them to determine the particular geologicalstructure involved in the area covered by the seismic survey.

In geophysical prospecting employing seismic methods one commonprocedure is what is known as reflection shooting. Information regardingthe geological structure is obtained by recording the times of arrivalof reflected waves at various points spaced from the shot point or thepoint where the impulse or elastic waves are created. In one of thecommonest methods of reflection shooting a series of geophones arelocated in line with the shot point at some distance from the shot pointwith the geophones spaced from each other. It will be apparent thatunder such conditions, the record on which is simultaneously recordedthe energy received at all of the geophones may be expected to show anarrival of a reflection at the most distant instrument from the shotpoint later than the arrival at the instrument or geophone nearest theshot point. This interval between the first and last reflection arrivalsis usually referred to as the stepout or step out time of the geophoneor detector array. It should be understood that step out time applies torefraction shooting as well as reflection shooting. In Fig. 5 of thedrawings there is illustrated a portion of the sensitized strip 54showing the traces 59, 60 and BI. An examination of these visualinspection traces indicates that all of them have a characteristic dipindicated at a, b and 0, respectively. The step out time is indicated bythe distance between the points a and 0. Since the correlation betweenthe traces 59, 6B and 6! is visual, very low frequency impulses whichhave a large step out time as indicated in Fig. of the drawings, aredifficult to correlate due to a persons limited lateral range of vision.In accordance with the present invention, it is possible to producerecords which are much easier to correlate even though the step out timeis large. In Fig. 6 of the drawings there is disclosed the same portionof the traces 59, 60 and 6| recorded in Fig. 5 except that the step outtime is greatly reduced as will be apparent from a comparison of Figs. 5and 6 of the drawings. This is accomplished in accordance with thepresent invention by manipulating the knob 28 of the change-speed gearmechanism 21 so as greatly to increase the speed of rotation of the drum16. Also in this connection, the filters 88, 8| and 82 are preferablywide band filters which will not attenuate the higher frequenciesproduced by the photo-electric cells 36, 31 and 38 at the high drumspeeds. Increasing the speed of the reproduction mechanism decreases therecord length over which a given step out time is recorded, as will beapparent from a comparison of Figs. 5 and 6 of the drawings. Thuscorrelation of the traces by the visual inspection method becomes easiersince the limited lateral range of vision does not interfere with thecorrelation of the traces of Fig. 6. In addition, the lateral extent ofeach reflection is reduced thereby facilitating correlation of thetraces as will be seen by comparing the traces 59, 68 and GI of Fig, 6of the drawings with the corresponding traces in Fig. 5.

In accordance with still another modification of the present nvenltion,correlation between the visual inspection type traces having a largestep out time may be greatly faciliated by displacing the photo-electriccells within the photo-electric cell assembly 33 relative to oneanother. Referring now to Fig. 4 of the drawings there is illustrated aportion of the reproduction device of Fig. 1 with the correspondingparts thereof designated by the same reference numerals. Thephoto-electric cells 38 and 39 are positioned in exactly the same way asin Figs. 1 and 2. However, the photo-electric cell 31 is displacedrelative to the photo-electric cells 38 and 39 in a direction oppositeto the film motion as indicated by the arrow in Fig. 4 of the drawings.With this arrangement impulses which are actually displaced from eachother as recorded in the variable area tracks I3 and I4 may be caused toappear substantially simultaneously when converted to visual inspectiontype traces 60 and BI respectively. Similarly, by displacingphoto-electric cell 36 in a direction opposite to the film motionrelative to photo-electric cell 31 a record such as is shown in Fig. '7of the drawings having a large step out time may be converted to thetype of record shown in Fig. 8 where the step out time is reducedapproximately to zero. The traces 59, 60 and 6| in Fig. 7 of thedrawings are designated by the same reference numerals in the modifiedrecord of Fig. 8. The amount of displacement between the photo-electriccells 31 and 38 and between the photo-electric cells 36 and 31 isdetermined by the apparent normal velocity at which the energy would beexpected to arrive at the geophones which initially re- 10 ceived theenergy recorded in the variable area tracks and converted by thePhoto-electric cells 36, 31 and 38 to electrical impulses which aresubsequently changed to a visual inspection type ofv trace or record. Itwill be apparent that "instead of displacing the cells 36, 31 and 38 inthe manner indicated in Fig. 4 of the drawings a similar result can beaccomplished by inserting electrical delay networks in the electricaloutput circuits of these cells. With this modification thephoto-electric cells can be positioned in the same manner as in Figs. 1and 2 of the drawings while producing the same results as thearrangement disclosed in Fig. 4 of the drawlngs.

It is well established in the reflection method of seismic prospectingthat the reflected energy usually decreases in frequency with time. Itis thus apparent that in the usual seismic energy recording operation,wherein the filters are pre-set at some predetermined value, optimumresults for all portions of the record are not realized. In accordancewith the present invention selective filtering may be accomplished totake care of the decrease in frequency of the reflected energy withtime. By changing the speed of the rotating drum l'6 of Fig. 1 during areproduction operation to conform with the change in frequency of thereflected energy with the time, the optimum adjustment for the filteringoperation to compensate for the variation of the frequency of thereflections can be obtained. In accordance with the present invention,therefore, the speed of rotation of the drum [6 during a reproducingoperation can be controlled in accordance with a predeterminedtimefunction. Thus the first part of the record during a reproductionoperation might be run slowly while the last part of the record is runmuch more rapidly to compensate for the decrease in frequency of thereflected energy. In Fig. 9 there is illustrated a portion of theapparatus of Fig. 1 having like components designated by the samereference numerals as in Fig. 1- and having means associated therewithfor controlling the speed of rotation of drum l6 in accordance with apredetermined time function. A suitable motor energized from any sourceof potential may be connected through a reduction gear mechanism 86 torotate a cam 81 which has a configuration such as to control the speedof rotation of drum IS in accordance with a predetermined time function.The cam 81 engages with the speed-change knob 28, moving it to the leftas viewed in Fig. 9 of the drawings as cam 81 is rotated in theclockwise direction indicated by the arrow, thereby increasing the speedof drum I6 during a reproduction operation. Preferably the motor 85 isstarted at a time corresponding to the time break appearing at thebeginning of a seismic record and the speed of drum I6 is increased,while a reproduction of the film or record i8 is made, to counteract forthe decrease in frequency of the reflected energy with time. In view ofthe detailed description included above the operation of the arrangementdisclosed in Fig. 9 of the drawings will be apparent to those skilled inthe art.

In some cases reflected energy in seismic prospecting of the reflectionshooting type does not decrease in frequency with time but instead mayvary in an irregular fashion. This may make it difiicult to predeterminethe proper time function for speed adjustment and it may be practicallyimpossible to design a cam such as the cam 81 which will compensate forthe frequency changes of the reflected energy with time. In accordancewith another modification of the present invention, illustrated in Fig.of the drawings, an arrangement is provided which will automaticallyadjust the speed of the rotating drum l6 and consequently the film speedin accordance with frequency variations of the reflected'energy.

The corresponding parts of Fig. 10 are designated by the same referencenumerals as in the preceding figures. Referring nowtoFig. IOof thedrawings, the photo-electric cell assembly 23 in addition to includingthe photo-electric cells 36,

37, 38 and 39; also includes photo-electric cells 90, 9|. and 92 whichare designated as the leading photo-electric cells since they arelocated a short distance ahead of the associated photoelectric cells 36,31' and 38, respectively, with reference to the direction of filmmovement indicated by the arrow inFig. IOof the drawings. In accordancewith the present invention,the output of the photo-electric cells 99, 9|and 92 is used automatically to adjust the speed of the drum [6 to theoptimum value in the manner described below. The leading photo-electricreplaces the synchronous motor 26 of Fig. 1 of cells 36, 31 and .38.

the drawings. Preferably the motor-99, which is illustrated as anordinary series field type motor the Speed regulations of which varieswith load, supplies the major portion of the driving power required forrotating the drum Hi. If the direct current voltage output Of thediscriminator'96 is in-the positive direction indicating that thefrequency of the signal energy is lower than the. center frequency ofthe filters 80, 8| and 82, the direct current motor 98 will be driven insuch a direction as to decrease the load on the motor 99, in other wordsauxiliary motor 98 will carry more of the load ofthe rotating drum I6and an increased speed of the drum will result.

The reflections will therefore be of a higher fre-- quency when they.intercept the. photo-electric put of the discriminator is in a negativedirection, the auxiliary ,motor98 will supply less driv-- ing powerandthe load on motor 99 will be in-.-

cells 99, 9| and. 92 are arranged to intercept the light energycontrolled by the variable area traces I2, l3 and M, respectively beforethis light is intercepted by the photo-electric cells 36, 3'! and 38.

combined that their responses are algebraically added. As indicated inFig, 10 of the drawingsthe photo-electric cells 90, 9| and 92 arearranged inthe normal step-out manner of the reflec tions in accordancewith the disclosure of Fig. 4.

of the drawings so that the reflected energy in all of the traces willoccur in phase insofar as the outputs of the photo-electric cells 90, 9|and 92 are concerned. By combining the energy outputs of thephoto-electric cells 90, 9| and 92 in the manner indicated thecombination disting'uishes between reflected energy and otherundesirable types of energy since the reflected energy has an additiveeffect, Whereas the random energy tends to cancel itself out.

In order to control the speed of rotation of the drum I6 in response tothe changes in frequency of the reflected energy, the electrical outputof the photo-electric cells 90, 9| and 92 is connected by suitableconductors 93 with an amplifier 94 which amplifies the electricalenergy, whereupon this amplified energy is supplied to a limiter95 Theoutputs a of the photo-electric cells 90, 9| and 92 are socreased-resulting in a decreased speed of the drum l6 and a lowerfrequency of the reflections when they intercept the photo-electriccells 36,

31 and38. The time constant of the discriminator network 90 ispreferably such that the speed adjustment determined by the leadingphoto electric cell bank comprisin photo-electric cells 90, 9| and 92will have been made before the reflections on the traces arrive at thephoto electric cellsSB, 31 and 38, respectively. With this arrangementit is obvious that the "leading to remove any amplitude modulations fromthe M;

amplified output of the photo-electric cells 99. 9| and 92. The constantamplitude output of the limiter 95 is in turn impressed upon adiscriminator 96 which distinguishes or interprets changes in frequencyin a manner well understood by those skilled in the art. Thediscriminator 96 is tuned to the center frequency of the filters 89, 8|and 82 so that frequencies higher than the center frequency will producea direct current output voltage of one polarity (negative) whilefrequencies lower than the center frequency will produce a directcurrent output voltage of the other polarity (positive), the magnitudeof the output voltages varying with the extent of frequency departurefrom the center frequency. The variable polarity direct current outputvoltage of the discriminator 96 is connected to a direct currentamplifier 91 which in turn supplies an auxiliary direct current motorschematically illustrated at 98.

bank of photo-electric cells comprising the cells 90, 9| and 92intercepts the energy far enough in advance to adjustthe speed and,therefore, the effective frequencyof the film or record ID for optimumresponse of the amplifier filter network to the reflection signals. Inview of the detailed description included above the operation of thearrangement disclosed in Fig. 10 will be obvious to those skilled in theart.

It should be understood that the present invention is not limited to thespecific constructions and arrangements described above and that changesand modifications may occur to those skilled in the art withoutdeparting from the spirit and scope of the present invention. It is,therefore, aimed in the appended claims to cover all such changes andmodifications.

I claim:

1. Apparatus for analyzing wave records of the type where theinformation contained thereon may be converted to variable frequencyelectrical energy, comprising a hollow rotatable drum formed oftransparent or light-transmitting material, a roller having its axis inspaced relationship relative to the axis of said drum, means foradjustably varying the position of said roller relative to said drum sothat wave records of varied lengths having their ends spliced togetherto form an endless belt may be rotatably supported on said drum androller, means for transmitting a beam of light through said belt anddrum, and a Th shaft; of the transmitting material, a roller having itsaxis in Similarly if the voltage out spaced relationship relative to theaxis of said drum, means for adjustably varying the position of saidroller relative to said drum so that seismographic records of variablelengths having their ends spliced together to form an endless belt maybe rotatably supported on said drum and roller, a source of lightadjacent to but outside of said drum, means for transmitting a beam oflight from said source through said belt and drum, and a photo electriccell assembly within said drum for converting said beam of light ascaused to vary by the information contained on said belt to variableelectrical energy.

3. Apparatus for analyzing seismographic records of the type having avariable area track thereon, comprising a hollow rotatable drum formedof transparent material, a roller having its axis in spaced parallelrelationship relative to the axis of said drum, means for adjustablyvarying the position of said roller relative to said drum so thatseismographic records of variable lengths having their ends splicedtogether in the form of an endless belt may be rotatably supported onsaid drum and roller, means for transmitting a beam of light throughsaid belt and drum so as to be intercepted by said variable area track,and a photo electric cell assembly within said drum for converting saidbeam of light as caused to vary by said variable area track to variableelectrical energy.

4. Apparatus for analyzing wave records of the type where theinformation contained thereon may be converted to variable frequencyelectrical energy, comprising a hollow rotatable drum formed oflight-transmitting material, means for supporting in readily releasablefashion said wave record on said drum for rotation therewith, a fixedsource of light outside said drum for transmitting a beam of lightthrough said wave record and drum, and a detachable photo electric cellassembly within said drum including a photo electric cell for convertingsaid beam of light as caused to vary by the information contained onsaid record to variable electrical energy.

5. Apparatus for analyzing seismographic records of the type where theinformation contained thereon is in the form of variable area trackswhich may be converted to variable frequency electrical energy,comprising a hollow rotatable transparent drum formed of methylmethacrylate, means for supporting in readily releasable fashion saidseismographic record on said drum for rotation therewith, a source oflight positioned so as to cause a beam of light to be transmittedthrough said record and said drum in a manner to intercept said variablearea tracks, and a photo electric cell assembly within said drum forconverting said beam of light as caused to vary by said variable areatracks to variable electrical energy.

6. The method of mounting for reproduction a seismic record of the typecomprising a length of an element having recorded thereon in the formofphonographically reproducible information the vibrations as received atdifferent points spaced from a source of disturbance in the earth,forming an endless belt of said length, placing said belt on areproducing device comprising a hollow rotatable drum and a rollerspaced therefrom, varying the position of said roller relative to saiddrum in dependence upon the length of said endless belt, and rotatingsaid drum to convert said phonographically reproducible information tovisual inspection type traces.

7. Apparatus for analyzing records of the type where the informationconseismographic tained thereon may be converted to variable frequencyelectrical energy, comprising a hollow rotatable drum formed oftransparent or lighttransmitting material having one open end, a rollerhaving its axis in spaced relationship relative to the axis of saiddrum, means for adjustably varying the position of said roller relativeto said drum so that seismographic records of varied lengths havingtheir ends spliced together to form an endless belt may be rotatablysupported on said drum and roller, means for transmitting a beam oflight through said belt and drum, and a photo electric cell assemblyextending into said drum through said open end for converting said beamof light as caused to vary by the information contained on said belt tovariable electrical energy.

8. Apparatus for analyzing seismographic records of the type where theinformation contained thereon may be converted to variable frequencyelectrical energy, comprising a hollow rotatable drum formed oftransparent or light-transmitting material having at least one open end,a roller having its axis in spaced relationship relative to the axis ofsaid drum, means for adjustably varying the position of said rollerrelative to said drum so that seismographic records of variable lengthshaving their ends spliced together to form an endless belt may berotatably supported on said drum and roller, a source of light adjacentto but outside of said drum, means for transmitting a beam of light fromsaid source through said belt and drum, and a photo electric cellassembly extending into said drum through said open end for convertingsaid beam of light as caused to vary by the information contained onsaid belt to variable electrical energy.

9. Apparatus for analyzing seismographic records of the type having avariable area track thereon, comprising a hollow rotatable drum formedof transparent material, a roller having its axis in spaced lparallelrelationship relative to the axis of said drum, threaded means foradjustably varying the position of said roller relative to said drum sothat seismographic records of variable lengths having their ends splicedtogether in the form of an endless belt may be rotatably supported onsaid drum and roller, means for transmitting a beam of light throughsaid belt and drum so as to be intercepted by said variable area track,and a photo electric cell assembly within said drum for converting saidbeam of light as caused to vary by said variable area track to variableelectrical energy.

10. Apparatus for analyzing elastic wave records of the type where theinformation contained thereon may be converted to variable frequencyelectrical energy, comprising a hollow rotatable drum formed oflight-transmitting material, said drum having at least one open end,means for supporting in readily releasable fashion said wave record onsaid drum for rotation therewith, a fixed source of light outside saiddrum for transmitting a beam of light through said wave record and drum,and a detachable photo electric cell assembly extending into said drumthrough said open end including a photo electric cell for convertingsaid beam of light as caused to vary by the information contained onsaid record to variable electrical energy.

11. Apparatus for analyzing seismographic records of the type where theinformation contained thereon is in the form of variable area trackswhich may be converted to variable frequency electrical energy,comprising a hollow roateaast i5 tatable transparent drum formed ofmethyl REFERENCES CITED methacrylate, means for supporting in readilyreleasable fashion said seismographic record on i g gg gg are of recordm the said drum for rotation therewith, a source of light positioned soas to cause a beam of light to be M UNITED STATES PATENTS transmittedthrough said record and said drum Number Name Date in a manner tointercept said variable area tracks, 1 872 Potter Jul 21 1938 and aremovable photo electric cell assembly ex- 2155507 Huber 1939 tendinginto said drum through one end thereof 2:243:73) Ems 1941 for convertingsaid beam of light as caused to m vary by said variable area tracks tovariable electrical energy.

JAMES E. HAWKINS.

